Pressure sensors have been utilized in various applications to measure either gauge pressure or absolute pressure. Many of these applications involve the measurement of pressure in unfavorable environments. The pressure sensor may be of a capacitive type or piezoresistive type. For example, an alumina ceramic capacitive sensor may comprise a thin, generally compliant ceramic sheet having an insulating spacer ring sandwiched between a thicker, non-compliant ceramic sheet. The first thin sheet or diaphragm is approximately 0.005 to 0.050 inches in thickness with a typical thickness of 0.020 inches. The thicker ceramic sheet has a thickness range between 0.100 to 0.200 inches. Those skilled in the art will appreciate that the thickness of the diaphragm is preferably dependent upon the diameter of the diaphragm. The spacer may be constructed of a suitable glass material. The apposed faces of ceramic disks are metallized by metals such as gold, nickel or chrome to create plates of a capacitor. A similar capacitive pressure transducer is described by Bell et al. in U.S. Pat. No. 4,177,496 (the '496 patent). Other capacitive pressure transducers similar to that described in the '496 patent are available and known in the art. A piezoresistive sensor typically utilizes a Wheatstone bridge, measuring changes in voltage and correlating the voltage changes to changes in sensed pressure. Either of these pressure sensor types may be utilized to measure the pressure of fluids in ultra-pure environments, however, there is a need for a non-contaminating pressure sensor.
Ultra pure processing of sensitive materials typically requires the use of caustic fluids. The susceptibility to contamination of the sensitive materials during the manufacturing process is a significant problem faced by manufacturers. Various manufacturing systems have been designed to reduce the contamination of the sensitive materials by foreign particles, ionic contaminants, and vapors generated during the manufacturing process. The processing of the sensitive materials often involves direct contact with caustic fluids. Hence, it is critical to deliver the caustic fluids to the processing site in an uncontaminated state and without foreign particulate. Various components of the processing equipment are commonly designed to reduce the amount of particular generated and ions dissolved into the process fluids, and to isolate the processing chemicals from contaminating influences.
The processing equipment typically includes liquid transporting systems that carry the caustic chemicals from supply tanks through pumping and regulating stations and through the processing equipment itself. The liquid chemical transport systems, which includes pipes, pumps, tubing, monitoring devices, sensing devices, valves, fittings and related devices, are frequently made of plastics resistant to the deteriorating effects of the caustic chemicals. Metals, which are conventionally used in such monitoring devices, cannot reliably stand up to the corrosive environment and will contaminate the process fluid for long periods of time. Hence, the monitoring and sensing devices must incorporate substitute materials or remain isolated from the caustic fluids.
While the processes must be very clean they often involve chemicals that are very aggressive. These could include for example harsh acids, bases, and solvents. The semiconductor industry has recently introduced processes, which make use of aggressive abrasives. Both the process equipment and the monitoring instrumentation must be impervious to the mechanical action of these abrasives.
Further, high reliability of process equipment instrumentation is a must. Shutting down a semiconductor or pharmaceutical line for any reason is costly. In the past, pressure transducers have commonly employed fill fluids separated by a thick isolating diaphragm to transmit pressure from the process to the sensor itself. The fill fluids are separated from the process by an isolator diaphragm of one sort or another. Failure of this isolator diaphragm and subsequent loss of fill fluid into the process can cause loss of product and require system cleaning before restarting operations. The isolating diaphragm will introduce significant and in some cases unacceptable pressure measurement errors. Eliminating the isolator diaphragm and fill fluid from the design is advantageous.
Also, the processing equipment commonly used in semiconductor manufacturing has one or more monitoring, valving, and sensing devices. These devices are typically connected in a closed loop feedback relationship and are used in monitoring and controlling the equipment. These monitoring and sensing devices must also be designed to eliminate any contamination that might be introduced. The sensing devices may include pressure transducer modules and flow meters having pressure sensors. It may be desirably to have a portion of the pressure sensor of the pressure transducer or flow meter in direct contact with the caustic fluids. Thus, the surfaces of the pressure sensor in direct contact with the caustic fluids should be non-contaminating. It has been found that porous materials allow the ingress and egress of caustic fluids through such materials. For example, ceramic materials are bound together with various glass like materials which themselves are easily attacked by the more aggressive corrosive materials. Hence, it is preferable that those portions of the pressure sensor in direct contact with caustic fluids be made of non-porous materials.
U.S. Pat. No. 4,774,843 issued to Ghiselin et al., describes a strain gauge having a single crystal sapphire diaphragm adhered to an aluminum oxide base. Ghiselin et al., indicates that the sapphire is adhered by means of a glass bonding material, epoxy or other adherent methods. Ghiselin et al., does not provide a further description of the glass bonding material or how the glass bond adheres to the sapphire and aluminum oxide base. However, the Ghiselin patent describes the glass bond as a low strength material that separates at strain points. The Ghiselin patent describes a change in geometry to reduce the strain point and thereby avoid the deficiencies of the low strength of the glass. U.S. Pat. No. 5,954,900 issued to Hegner et al. describes problems with using a glass to bond to an aluminum oxide ceramic part. The Hegner et al. patent describes the use of alumina as the joining material to alumina ceramic. The devices described by Hegner et al., and Ghiselin et al., are believed to be limited to effective operable temperatures below 400° C. Thus, the reliability of the sensors described by Hegner et al., and Ghiselin et al. patents, decreases as temperatures exceed 400° C. Glasses with low melting points have low strength and low mechanical stability. Further these glasses generally have problems in developing uniform bonds. All these characteristics lead to a sensor with lower that optimal repeatability and hysteresis. Hence, there is a need for a pressure sensor having a non-porous surface that is bonded to the base with a high strength bond, wherein the bond between the non-porous material and the base is stable at temperatures in excess of 400° C.
It has also been found that Electromagnetic and Radio Frequency Interference (EMI and RFI respectively) degrade the performance of piezoresistive sensors. A conductive shielding layer cannot be positioned directly between a silicon layer (on which the Wheatstone bridge is formed) and the sapphire because of the epitaxial construction of silicon on sapphire. A conductive shielding layer on the outside of the sapphire is not preferred when the outside of the sapphire is positioned in contact with the caustic fluids. Hence, a need exists for a non-contaminating pressure sensor that blocks the EMI and RFI from affecting the sensing element formed on a non-exposed surface of the pressure sensor.
High temperature processes (600° C. to 1200° C.) are desirable to join single crystal materials such as sapphire or silicon carbide to other single or polycrystalline ceramics via brazing, glassing, and diffusion bonding because they make strong, high yield, stable joints. Where a high temperature process is used the usual methods of making an electrical connection to a semiconductor device on the single crystal or ceramic substrate (typically silicon, but can also include gallium arsenide) can no longer be used. The two most common methods of connection are 1) wire bonding and 2) conductive epoxy joining. With wire bonding a gold or preferably aluminum metal layer must be first deposited on the silicon. At high temperatures gold and aluminum rapidly diffuse into the silicon. Once diffused these material layers no longer form a suitable surface for the wire to bond to.
Metal films that survive a high temperature environment consist of an adhesion layer, such as titanium, which is followed by a diffusion barrier. The diffusion barrier for high temperature processes is a refractory metal such as molybdenum, iridium, niobium, tantalum, tungsten, or osmium. These metals will build up resistive oxides between the conductor and the epoxy over time. For piezoresistive sensors this creates a stability problem. Pre-cleaning the joint prior to applying epoxy slows but does not prevent the formation of the oxide layer. Solder will also not adhere to refractory metals.
One approach for affixing pins or leads to a semiconductor substrate is to braze pins in place as illustrated in FIGS. 19A and 19B, respectively. In particular, FIG. 19A is an example of a prior art brazing of a nail head pin to a substrate while FIG. 19B is an example of a prior art brazing of a headless pin to a substrate. FIG. 19A illustrates a single crystal substrate 200 with a pin 210 that is brazed thereon. Pin 210 includes a pin shaft 212 and a pin head 214 that provides a greater bonding surface area with substrate 200. A braze 216 is applied to the sides of pin head 214. As illustrated via a stress fracture 218, on a thin single crystal material stress fracture becomes a significant problem. As nail head pin 214 is brazed to the flat surface of substrate 200, substrate 200 develops tensile stress as the melted braze hardens (or freezes) and contracts. The tensile stress concentrates at the periphery of pin head 214 and promotes fracturing in the substrate.
One solution to the stress fracturing is to find a metal that matches the thermal expansion rate of the substrate crystalline material. Unfortunately, crystals such as sapphire have different expansion rates in different directions. More seriously metals do not have constant expansion rates over large temperature ranges. They typically expand at a much faster rate at high temperatures than at room temperature. An alloy that matches the temperature coefficient of the crystal at room temperature will have a much higher expansion at 800° C. or 900° C.
As illustrated in FIG. 19B, another approach to minimize stress in substrate 200 is to minimize the cross sectional area of the joint between substrate 200 and a headless pin 210. The headless pin helps to form a very small cross section joint with substrate 200, however the newly formed joint is susceptible to high, localized stress from manipulation of the pin (back and forth—as shown by arrow 220) during subsequent manufacturing steps. A butt joint of this type is considered an unreliable geometry because of the small attachment area and susceptibility to alignment problems.
The present invention meets these and other needs that will become apparent from a review of the description of the present invention.